Field
Embodiments related to an electrically activated lens filter with an electro-optic portion having an electric field gradient with radial and circumferential symmetry are disclosed. More particularly, an embodiment related to a lens filter that may be integrated in a camera module to provide an aperture stop, is disclosed.
Background Information
Camera modules have been incorporated in a variety of consumer electronics devices, including mobile devices such as smart phones, mobile audio players, personal digital assistants, and other portable and desktop computers. A typical camera module includes an optical system used to collect and transmit light from an imaged scene to an imaging sensor. The optical system generally includes at least one lens associated with one aperture stop. The lens collects and transmits light. The aperture stop limits the light collected and includes an aperture through which light is transmitted. The aperture is therefore termed the stop aperture, or alternatively, the camera pupil. The effective diameter of the stop aperture combined with the lens focal length determines the “F number” of the lens. A lens with a lower F number produces a brighter image than a lens with a larger F number and, as a result, reduces the image noise in a low light scene. However, as the F number is reduced, the lens depth of field decreases and, as a result, lens aberrations increase. Thus, there is an optimal aperture size, dependent on the lens and the scene being imaged, to minimize image noise and maximize image resolution.
In most portable consumer electronics devices, minimizing device profile is an important design goal. Accordingly, device profile limitations generally prohibit the use of an iris diaphragm as a variable aperture stop. Another way to control the amount of light admitted through the lens to balance image brightness and resolution is to use an electro-optic aperture. Such devices may be sized to fit within the space constraints of portable consumer electronics devices. An electro-optic aperture may include an electro-chromic (EC) medium that attenuates light that is passing through the aperture, in response to a voltage being applied to a pair of transparent conductive layers that sandwich the EC medium. One of the transparent conductive layers may be patterned to include a void in a central portion, so as to form a ring-like aperture stop with an inner aperture area that remains transparent when the EC medium is energized and an outer stop that becomes dark, thereby yielding in effect a smaller pupil size. With this approach, the patterned transparent conductive layer creates a radially uniform electric field in the EC medium, and thus, uniform opacity across the outer stop area of the ring-like aperture stop. Since voltage may be applied at a single location around a circumference of each transparent conductive layer, the electric field may vary substantially in a circumferential direction, with a maximum field located at a point of contact and a minimum field located opposite from the point of contact. This electric field, which may be radially uniform (no-gradient from an outer edge to a center location) and circumferentially non-symmetric (no symmetry and/or uniformity of an electric field about a central axis) may generate opacity with a “top hat” light transmission profile, such that light transmission drops off sharply between the aperture and the stop regions, and varies in a circumferential direction.